In prior art vacuum fluorescent devices, anodes coated with fluorescent material usually a phosphor are laid down in a desired arrangement on a glass substrate. A filament and control grid are mounted on the substrate and a transparent cover plate usually of glass is used to enclose the anodes, grid, and filament. The cover is hermetically sealed to the substrate. The assembly is then evacuated and sealed in a conventional manner. Electrical connections enable selective application of electrical potential to excite the phosphor on the anode into fluorescence.
The filament is heated to "boil off" electrons. These electrons are accelerated by an electrical potential between the filament and anode. The accelerated electrons strike the phosphor causing it to fluoresce.